PROJECT SUMMARY/ABSTRACT Binaural hearing allows listeners to better understand speech in noise and localize sounds. This proposal aims to develop a clearer understanding of the mechanisms that contribute to poorer binaural hearing abilities in patients with cochlear implants. The proposed experiments investigate the effects of poor hearing outcomes in one or both ears on speech perception and computational modeling of binaural processing in the brainstem. Abnormal speech perception with poor speech outcomes in one or both ears may contribute to poorer speech understanding in noise. Aim 1 will investigate differences in speech perception in cases of simulated or actual asymmetrical hearing, when speech understanding is poor in one or both ears. Factors that affect cochlear implant patients, namely the fidelity of temporal information conveyed by cochlear implant electrodes, will be simulated in younger and older listeners with normal hearing to account for aging effects. By testing participants with normal hearing and cochlear implants, results will have important implications for changes in perception with disease and perception with normal hearing. Poor encoding of binaural cues with asymmetrical hearing may result in reduced sensitivity to binaural cues and poorer sound localization performance. Aim 2 will develop a computational model of binaural encoding in the brainstem where one or both sides are deteriorated as in hearing impairment to predict the effects of hearing loss and cochlear implant stimulation on binaural processing. This approach will allow for an explanation of perceptual results in terms of the physiological mechanisms involved in the brainstem. Specific factors that affect patients with hearing loss and patients that use cochlear implants can be modeled and assessed separately to account for changes that occur along the binaural pathway. The training program will result in the development of the following specific scientific skills: (1) sound processing for cochlear implant simulations and speech perception experiments, (2) programming cochlear implants for direct connect experiments, (3) development and testing of computational models of binaural processing, and (4) ability to make electrophysiological recordings and understand their limitations. Each of these skills is critical to the applicant?s development as a scientist and future career in binaural hearing research. With these and other existing research skills, it will be possible to establish an independent and productive line of research concerning binaural hearing outcomes for adults with bilateral cochlear implants and normal hearing. Good binaural hearing improves the lives of patients. The results of the projects in this proposal will shed light on the perceptual and physiological problems that limit binaural hearing abilities in listeners with bilateral cochlear implants. Ultimately, these results will be important in understanding the limitations to binaural hearing for cochlear implants and improved cochlear implant processing strategies.